Preheat oven to 375°F (190°C) and grease a 9x13 inch baking dish.
Cook rigatoni in a large pot of salted boiling water until 2 minutes short of al dente; drain and set aside.
In a large skillet over medium-high heat, sauté the ground beef and diced onion until meat is browned and onion is translucent. Drain excess fat.
Add minced garlic, oregano, salt, and pepper to the skillet; cook for 1 minute until fragrant.
Pour marinara sauce into the meat mixture and simmer on low heat for 10 minutes.
Toss the par-cooked rigatoni with the meat sauce until evenly coated.
Layer half of the pasta mixture into the prepared baking dish and sprinkle with 1 cup of mozzarella cheese.
Top with the remaining pasta mixture, followed by the remaining mozzarella and the grated parmesan cheese.
Bake for 20-25 minutes until the cheese is melted, bubbling, and slightly golden.
Remove from oven and garnish with fresh parsley before serving.